This invention relates generally to resist layer structures and more particularly to high sensitivity positive resist layers useful in high energy radiation exposure processes.
The formation of positive resist masks from layers of radiation degradable polymers is described, for example, by Haller and Hatzakis in U.S. Pat. No. 3,535,137. In this process a radiation degradable polymer layer is coated on a substrate and is subjected to patternwise exposure to high energy radiation such as, for example, x-rays, nuclear radiation, and electrons. The irradiated regions of the polymer suffer a decrease in molecular weight and thereby become more soluble. A developer is then used to preferentially remove the irradiated portions of the layer. The substrate can then be subjected to an additive or subtractive process such as metallization or etching with the remaining portions of the resist layer protecting the substrate from the processing.
Although it is possible to employ developers which attack only the exposed portions of the resist and leave the unexposed portions almost intact, it has been found that much higher resist sensitivity can be obtained if a solvent is employed which attacks both the exposed and unexposed portions of the resist but preferentially attacks the exposed portion. The resist layers are applied in sufficient thicknesses so that the unexposed portions of the resist layer retains a sufficient thickness for further processing.
The sensitivity of the resist layer can be defined by the ratio S/S.sub.0 where S is the solubility rate of exposed resist for a given exposure dosage and S.sub.0 is the solubility rate in angstroms per minute for the unexposed resist. Generally a S/S.sub.0 ratio of at least about 2.0 is required for most processes so that a sufficient unexposed resist layer thickness remains after development. One way to increase the ratio is to increase the dosage. However, this has the disadvantage of slowing down the exposure process particularly when the exposure is accomplished using a scanning beam of radiation. For example, in the formation of integrated circuits or exposure masks, one process which is employed is a liftoff process in which a patterned relief layer of resist is first formed on a substrate. A layer of material, such as a metal for integrated circuit conductor lines or an opaque masking material for mask fabrication is coated over the resist layer and the exposed portions of the substrate. The resist layer is then stripped off and takes with it the overlying material to leave only the pattern of material in direct contact with the substrate. Such a process is described, for example, by Hatzakis, Electron Resists For Micro Circuit And Mask Production, Journal of the Electro Chemical Society, Volume 116, No. 7, pages 1033 to 1037, July 1969 and by Hatzakis and Broers, Record of the Eleventh Symposium on Electron, Ion and Laser Beam Technology, pages 337 to 344. The described process takes advantage of the natural undercutting of the resist during high energy exposure such that the developed resist pattern is wider at the bottom then the top. This provile aids in forming a discontinuity between the portions of material which are on the substrate surface and the portions which cover the resist. This dicontinuity is needed in order to permit the resist stripping solution to attack the unexposed resist and remove it along with the overlying material. The resist layer thickness required for a metal lift-off process, for example, must be in the ratio of a minimum of about 1.5/1 resist thickness to metal thickness to avoid bridging of metal between the portion on the substrate and the portion covering the resist. Therefore, the loss of unexposed resist during development must be limited. In other words, the solubility ratio S/S.sub.0 must be maximized. This can be done by increasing exposure times to create a greater molecular weight differential between the exposed and unexposed resist. However, this has the effect of slowing down the exposure process.
Another factor involved in the use of high energy radiation to exposed resist layers is the fact that there are certain advantages to the use of, for example, beams of increasing energy, for example, 10 to 50 kilovolts (KV). This has the advantage of decreasing writing times for a resist because higher amps/per square centimeter can be produced by the electron gun and the higher energy beams also provide more back scatter electron signal for better registration and detection. However, it has been found that for any given resist thickness employed, the higher energy beams produce less electron scattering and undercutting of the resist such that excessive exposure times would be required to obtain the desired undercut profile by exposure alone.